When Inari returns her powers to Uka, why doesn't she turn back to looking like Sumizome? Remember: Uka gave her those shapeshifting powers to enable her to return back to her original shape since wishes can not be reverted, so the wish to look like Sumizome would still be in effect.
Most likely not all of it was returned, and Inari subconsciously kept a small fragment of it, just enough to keep herself in her original form but not enough for them to know it is there. In such a case they would have only noticed it if they had a reason to suspect something was wrong, but because Uka got used to Inari looking like Inari rather than Sumizome (Inari was visiting the shrine notoriously often in the past), she didn't think of it and thus did not notice. Additionally, the leftover fragment of Uka's power would be the reason why Inari started either siphoning the power out of Uka again or developing her own.
I will add that Inari's default form after the half-ascension was always Inari, meaning that the gift of divine power was intended for Uka to grant her own 'second' wish, and the rest was purely window-dressing. Well, Show me a time she accidentally turned into Sumizome after that second visit.
It is shown that Uka can take the shape of a human being, as she did to help a young Touka when he got lost on shrine grounds. So why doesn't she do this after Inari returns her powers? She would still have been able to talk with Inari that way, even if only for a while.
She may have taken on a more human-looking form, but the reason Touka could see her was his spiritual awareness. Inari's awareness is(was) at a lower level(or unawakened), so going with the above, she needed a dose of Uka's powers in order to lift it to the level where she could perceive spirits the same way her brother could.